Enhancing Flame-Retardant Properties of Polyurethane Aerogels Doped with Silica-Based Particles

Highlights Aerogels doped with SiO2 particles derived from rice husks are obtained by a freeze-drying method. SiO2 particles provide a reduced-flammability advantage over undoped PUR aerogels. Increasing SiO2 content significantly enhances flame-retardancy in PUR aerogels. Abstract In this work, polyurethane (PUR) aerogels doped with different SiO2 particles, derived from a renewable source, were successfully synthesized, and the effects of SiO2 content on the properties of PUR aerogels were investigated. Specifically, three types of SiO2-based particles obtained from rice husk through different procedures were evaluated to enhance the thermal stability of the composites with special attention given to flame-retardant properties. With the optimal SiO2 particles, obtained through acid digestion, the influence of their content between 0.5 and 3 wt.% on the physicochemical characteristics of the synthesized aerogels was thoroughly examined. The results showed that increasing the doping agent content improved the lightness, thermal stability, and flame-retardant properties of the resulting PUR aerogels, with the best performance observed at a 2 wt.% doping level. The doped aerogel samples with non-modified SiO2 particles significantly enhanced the fire safety performance of the material, exhibiting up to an eightfold increase in flame retardancy. However, modification of the SiO2 particles with phytic acid did not slow down the combustion velocity when filling the aerogels. This research highlights the promising potential of doped PUR/SiO2 aerogels in advancing materials science and engineering applications for withstanding high temperatures and improving fire safety.


Introduction
Making buildings more efficient in terms of fire resistance in construction materials is essential to minimize fire spread and extend evacuation time.Incorporating fire-resistant materials, such as insulation panels and coatings, is key in slowing down flame propagation and reducing the emission of harmful toxic smoke [1].
Conventional insulation materials, such as foams (colloidal systems like sols, gels, and aerogels), are widely used for insulation and have advantages such as being widely available, affordable, and easy to install.However, they also have disadvantages such as low fire resistance (requiring the incorporation of flame-retardant additives), potential toxic emissions, and limited energy efficiency.In this sense, it is important to note that combustible insulation materials are responsible for most fatal fires.The widespread use of these types of commercial materials has generated an urgent need to develop safer and Gels 2024, 10, 465 2 of 16 more efficient alternatives, such as new fire-resistant insulation materials for buildings [2].These materials are essential for preventing the spread of fire and protecting the lives of building occupants.Ongoing technological advancements and research into new materials are leading to the development of safer and more efficient options in terms of fire resistance, providing enhanced protection in case of emergencies.Depending on their nature, flameretardant systems can act either physically (by cooling, forming a protective layer, or diluting fuel) or chemically (through reaction in the condensed or gas phase).They can interfere with the various processes involved in polymer combustion (heating, pyrolysis, ignition, and propagation of thermal degradation) [3,4].
Aerogels, highly porous materials with low density (sometimes likened to solid foams due to their structural similarities), are renowned for their exceptional thermal insulation properties [5].Their historical and commercial significance spans nearly a century, during which they have undergone complex development but demonstrated remarkable potential across diverse applications [6,7].Most aerogels are synthesized using the sol-gel method, a green synthesis technique that is applied under mild conditions and with eco-friendly solvents such as water and alcohols [8][9][10].Recent focus has been on organic aerogels, particularly those primarily composed of polyurethane polymer chains (PUR aerogels), aimed at producing materials with exceptionally low conductivity and density [11][12][13].Key attributes of organic aerogels include high insulation capacity, stemming from their low thermal conductivity effectively reducing heat transfer.Additionally, they boast low density and high porosity, affording a substantial surface area that enhances insulation performance.Furthermore, aerogels exhibit flexibility, enabling them to conform to various shapes and surfaces for convenient installation.However, the high flammability of most organic polymeric materials presents a significant challenge, resulting in the generation of substantial smoke, toxic gases, heat, and melting drips during combustion.Consequently, these factors contribute to considerable damage to human lives and properties annually, underscoring safety concerns across various fields of application [14].
To improve the fire-retardant behavior of PUR aerogels (and other hybrids synthesized via sol-gel in environmentally friendly conditions) and to reduce the release of plentiful heat and toxic smoke, different flame-retardant fillers can be incorporated into the aerogel matrix.These flame-retardant dopants should not negatively affect the synthesis process.Some additives that improve flame resistance and suppress smoke include carbon nanotubes [15], clays [16], lignin [17], phosphorus [18], and silica (SiO 2 ) particles [19].Among them, SiO 2 particles stand out due to their small size and high surface area, which allow them to interact with the PUR matrix and enhance its structure and properties [20].
During the synthesis of SiO 2 -doped PUR aerogels, the concentration and dispersion of SiO 2 particles, as well as the manufacturing process, can significantly influence the extent of property enhancement [21,22].Generally, an increase in SiO 2 content leads to enhanced mechanical strength, thermal stability, and fire resistance of the aerogel [19,23,24].However, an excess of SiO 2 particles can negatively affect the porosity and density of the aerogel, which can reduce its thermal insulation performance.Therefore, it is important to establish an appropriate balance in the content of SiO 2 particles to achieve the best final properties of PUR aerogels [21,25].
The objective of this study was to investigate the influence of three types of SiO 2 particles as doping agents in PUR aerogels, with a particular focus on flame-retardant properties.A novel aspect of this research is the use of SiO 2 additives derived from natural rice husk (RH) as dopants, which are incorporated into the polymer matrix.This innovative approach not only reduces the environmental footprint by using a low-cost and widely available crop residue, but also enhances the sustainability of SiO 2 production, contributing to the circular economy.The study details the effect of particle content on the physicochemical properties and performance of the synthesized aerogels.

Impact of Silica Particles' Nature as Dopant of PUR Aerogels
The incorporation of SiO 2 particles of a different nature depending on the extraction procedure (AD and SG at 2 wt.%) into the polymeric structure of PUR aerogels was evaluated in detail.Table 1 summarizes the SiO 2 -doped aerogels' characterization results obtained in terms of density, thermal conductivity, Young's modulus, and glass transition temperature (T g ).The "PUR" sample corresponds to the aerogel sample that is free of SiO 2 particles, commonly known as a blank sample.As can be seen, the densities of the doped samples increase such that PUR_AD 2 < PUR_SG 2 , while the low thermal conductivity values obtained for the doped PUR aerogels remain practically constant regardless of the type of particles incorporated.On the other hand, the incorporation of SiO 2 _AD 2 particles lightly increases the surface area and total pore volume of the resulting aerogel compared with the undoped one, contrary to what was observed when incorporating the SiO 2 _SG 2 particles, which resulted in aerogels with a very low surface area.As a result, higher density in the doped samples and lower surface area correspond to increased sample hardness, as indicated by Young's modulus.The T g results for all samples at around −50 • C are consistent with the typical PUR structure [26].
The SEM results depicted in Figure 1 confirm the earlier findings.The porous structure of the PUR_SG 2 aerogels is observed to be closing, resulting in a more compact appearance consistent with the surface area, pore volume, and density values already mentioned.This fact is further supported by the pore size distribution (PSD) obtained by a Hg porosimeter, which indicated pore sizes ranging from 3 to 300 µm for both samples, as illustrated in Figure 2.Moreover, as an example, the elemental mapping and EDS analysis of the PUR_AD 2 sample are presented in Figure S1, showcasing the well-dispersed SiO 2 particles throughout the polymer.
The influence of the nature of the SiO 2 particles incorporated into the structure of PUR aerogels on their thermal degradation is illustrated in Figure 3 (TGA curves corresponding to the synthesized SiO 2 -doped aerogel samples).As can be observed, similar profiles are found for the different aerogels, including the undoped one.PUR aerogel samples exhibit thermal stability of up to 230 • C, wherein they undergo complete loss of any water or residual solvents.Subsequently, between 230 and 440 • C, the most significant mass loss (>90 wt.%) occurs due to the decomposition of a PUR network.Finally, from 440 to 700 • C, the decomposition of ester groups takes place, leaving approximately 1 wt.% remaining [27,28].
To explore the use of synthesized SiO 2 -doped aerogels as fireproof materials in the construction sector, their flame-retardant capability was studied.The combustion velocity was calculated after the total time, ignition, and propagation (13 s) (Figure 4).As can be observed, both doped aerogels with AD and SG at 2 wt.% reduced the combustion velocity compared with the undoped aerogel.Interestingly, the presence of AD 2 particles in the polymeric matrix significantly mitigated the combustion velocity of the aerogel, and it does so up to more than six times compared with PUR_SG 2 , resulting in a velocity as low as 0.07 ± 0.01 mm•s −1 .Therefore, these results strongly suggest that SiO 2 particles obtained through AD of RH and subsequent calcination (PUR_AD 2 ) demonstrates superior performance of flame retardancy when acting as a doping agent for PUR compared with the sample obtained through alkaline extraction of the RH ash, followed by sol-gel precipitation (PUR_SG 2 ).Considering the physicochemical characterization of the particles (Table 4), the results could be attributed to the fact that the alkaline extraction and sol-gel precipitation process generates SiO 2 particles with the presence of metallic impurities, particularly alkaline oxides [29], which could be related to the formation of more compact and, therefore, more dense aerogels, as demonstrated previously.Nevertheless, it is noteworthy that acid leaching effectively removed most inorganic impurities, resulting in highly pure SiO 2 (see FTIR results; Figure S2).In any case, the obtained flame-retardant results are in perfect agreement with the thermal stability observed from the thermogravimetric analyses conducted on each of the SiO 2 particles (Figure S3).The combustion of polymeric materials initiates upon the application of heat, leading to thermal degradation.The resultant degradation byproducts become superheated and congregate to create bubbles.One strategy for flame retardancy involves reducing the rate of bubbling.In this sense, SiO 2 particles reduce the flammability of polymer materials by suppressing the intense bubbling phenomenon that occurs during the degradation process in combustion [30,31].Based on the achieved results, the SiO 2 _AD 2 particle type was selected as a dopant for PUR aerogels with the aim of enhancing the flame-retardant properties of the resulting materials.

Effect of the Content of Acid-Extracted SiO 2 Particles in PUR Aerogels
After selecting AD particles as the best SiO 2 doping agent, the impact of their concentration in the aerogels was evaluated within the range of 0.5 to 3 wt.%.The findings related to density, thermal conductivity, and Young's modulus are outlined in Table 2.The influence of the nature of the SiO2 particles incorporated into the structure of PUR aerogels on their thermal degradation is illustrated in Figure 3 (TGA curves corresponding to the synthesized SiO2-doped aerogel samples).As can be observed, similar profiles are found for the different aerogels, including the undoped one.PUR aerogel samples exhibit thermal stability of up to 230 °C, wherein they undergo complete  To explore the use of synthesized SiO2-doped aerogels as fireproof materials in the construction sector, their flame-retardant capability was studied.The combustion velocity was calculated after the total time, ignition, and propagation (13 s) (Figure 4).As can be observed, both doped aerogels with AD and SG at 2 wt.% reduced the combustion velocity compared with the undoped aerogel.Interestingly, the presence of AD2 particles in the polymeric matrix significantly mitigated the combustion velocity of the aerogel, and it does so up to more than six times compared with PUR_SG2, resulting in a velocity as low as 0.07 ± 0.01 mm•s −1 .Therefore, these results strongly suggest that SiO2 particles obtained through AD of RH and subsequent calcination (PUR_AD2) demonstrates superior performance of flame retardancy when acting as a doping agent for PUR compared with the sample obtained through alkaline extraction of the RH ash, followed by sol-gel precipitation (PUR_SG2).Considering the physicochemical characterization of the particles (Table 4), the results could be attributed to the fact that the alkaline extraction and sol-gel precipitation process generates SiO2 particles with the presence of metallic impurities, particularly alkaline oxides [29], which could be related to the formation of more compact and, therefore, more dense aerogels, as demonstrated previously.Based on the achieved results, the SiO2_AD2 particle type was selected as a dopant for PUR aerogels with the aim of enhancing the flame-retardant properties of the resulting materials.

Effect of the Content of Acid-Extracted SiO2 Particles in PUR Aerogels
After selecting AD particles as the best SiO2 doping agent, the impact of their concentration in the aerogels was evaluated within the range of 0.5 to 3 wt.%.The findings related to density, thermal conductivity, and Young's modulus are outlined in Table 2.The correlation between SiO2 content and density values is evident, supported by observations from SEM micrographs (Figure 5).These images reveal increased porosity at a higher dopant content, leading to a more porous structure in samples with over 0.5% SiO2 content.This contrasts with the denser nature of the 0.5 wt.% doped sample, as confirmed by density results.As anticipated, lighter samples (PUR_AD2 and PUR_AD3) The correlation between SiO 2 content and density values is evident, supported by observations from SEM micrographs (Figure 5).These images reveal increased porosity at a higher dopant content, leading to a more porous structure in samples with over 0.5% SiO 2 content.This contrasts with the denser nature of the 0.5 wt.% doped sample, as confirmed by density results.As anticipated, lighter samples (PUR_AD 2 and PUR_AD 3 ) indicate a higher surface area and greater total intruded pore volume.However, it is remarkable that the PUR_AD 2 sample contains a higher number of small-sized pores compared with the PUR_AD 3 aerogel, which has an open surface and lacks large-sized pores.This fact can be corroborated in Figure 6, where the pore size distributions (PSDs) of PUR aerogels doped with SiO 2 _AD particles at different percentages are represented.The PSD shows how the peak of the Gaussian representation shifts to lower values as the metallic content increases, until it reaches approximately 2%.
Moreover, although the thermal conductivity values seem to remain relatively unchanged across the doped samples, all of them exhibit enhanced insulating properties compared with the undoped sample.Furthermore, an increase in SiO 2 content corresponds to a rise in Young's modulus, which was determined from the linear region of the stress-strain curves, indicating an improvement in the mechanical properties of the samples [32].
The study aimed to evaluate the influence of SiO 2 content on fire safety enhancement.Specifically, the impact of SiO 2 content on PUR aerogels was examined in relation to combustion velocity (Figure 7A).Experimental images of the residual char post-sample combustion are presented in Figure 7B.The results indicate an inverse relationship between SiO 2 content and combustion rate within the studied range.However, a SiO 2 content exceeding 2 wt.% does not result in a reduction in combustion velocity.According to the results, it seems that the improvement in the flame-retardant properties of the synthesized aerogels is closely connected to the porosity of the samples.According to the literature for other fillers [33], aerogels doped with SiO 2 fillers exhibited smaller sizes and more uniform pores (see Figures 1 and 5, PUR undoped vs. PUR_AD 2 ).This was attributed to the addition of fillers increasing the viscosity of the precursor solution, acting as nucleation agents.This contributed to an ordered structure, resulting in enhanced mechanical and thermal properties, thereby making flame propagation more challenging.Therefore, the comparable combustion speeds of the PUR_AD 2 and PUR_AD 3 samples are attributed to their similar surface area and pore volume, as previously mentioned, despite their differing morphologies.Therefore, a SiO 2 content of 2 wt.% of SiO 2 _AD is deemed the optimal particle concentration for incorporation into the polymer structure to minimize the flammability of aerogel samples.Following common practice in the literature for this type of materials [34,35], different images were recorded with a thermal camera where the sample emits an amount of infrared radiation as a function of its temperature (Figure S4).The images reveal the notable difference in combustion when the samples contain a SiO 2based additive (2 wt.%) extracted by AD from RH, showing the char residues generated in each case.PUR undergoes thermal degradation through a well-known mechanism [36,37] that involves its dissociation into primary amines, olefins, and carbon dioxide.This process typically occurs when PUR is exposed to elevated temperatures, leading to the breakdown of its polymeric structure (Figure S5).With the addition of a SiO 2 -based additive, the samples tend to form an intumescent char layer after combustion.The high quality of a SiO 2 layer form would act as a good physical barrier to resist the transfer of heat and combustible gases, resulting in the enhancement of flame-retardant performance.However, as other authors have observed, SiO 2 excess could suppress the intumescent process of the carbonization layer, resulting in a decrease in the synergistic effect on the flame retardancy and smoke suppression properties of the coatings [38].
Gels 2024, 10, x FOR PEER REVIEW 7 of 18 indicate a higher surface area and greater total intruded pore volume.However, it is remarkable that the PUR_AD2 sample contains a higher number of small-sized pores compared with the PUR_AD3 aerogel, which has an open surface and lacks large-sized pores.This fact can be corroborated in Figure 6, where the pore size distributions (PSDs) of PUR aerogels doped with SiO2_AD particles at different percentages are represented.
The PSD shows how the peak of the Gaussian representation shifts to lower values as the metallic content increases, until it reaches approximately 2%.Moreover, although the thermal conductivity values seem to remain relatively unchanged across the doped samples, all of them exhibit enhanced insulating properties compared with the undoped sample.Furthermore, an increase in SiO2 content corresponds to a rise in Young's modulus, which was determined from the linear region of the stress-strain curves, indicating an improvement in the mechanical properties of the samples [32].The combustion study suggests that enhancing the fire resistance of samples can be feasibly achieved by incorporating additives derived from natural and cost-effective sources, such as SiO 2 particles obtained from RH, as opposed to their respective counterparts.

Effect of the Modification of SiO 2 Particles with a Flame-Retardant Agent
It is widely described in the literature that PA is a bio-based and environmentally friendly compound that is used as a flame-retardant agent in different matrices [39,40].By using QAS as a crosslinker, an effective anchoring of PA to SiO 2 _AD particles was achieved (as demonstrated by FTIR analysis; see Figure S2).
In agreement with the findings reported in the previous subsection, the selected content of AD@QAS-PA for the preparation of PUR samples was 2 wt.%.Physicochemical properties (Table 3) and the SEM results (Figure 8) show that the incorporation of SiO 2 _AD@QAS-PA 2 particles resulted in aerogels with a very low surface area and high density.As a result, this increased the aerogel hardness, as indicated by Young's modulus.The porous structure of the PUR_AD@QAS-PA 2 aerogels is observed to be closing, resulting in a more compact appearance consistent with the surface area, pore volume, and density values already mentioned.The study aimed to evaluate the influence of SiO2 content on fire safety enhancement.Specifically, the impact of SiO2 content on PUR aerogels was examined in relation to combustion velocity (Figure 7A).Experimental images of the residual char post-sample combustion are presented in Figure 7B.The results indicate an inverse relationship between SiO2 content and combustion rate within the studied range.However, a SiO2 content exceeding 2 wt.% does not result in a reduction in combustion velocity.According to the results, it seems that the improvement in the flame-retardant properties of the synthesized aerogels is closely connected to the porosity of the samples.According to the literature for other fillers [33], aerogels doped with SiO2 fillers exhibited smaller sizes and more uniform pores (see Figures 1 and 5, PUR undoped vs. PUR_AD2).This was attributed to the addition of fillers increasing the viscosity of the precursor solution, acting as nucleation agents.This contributed to an ordered structure, resulting in enhanced mechanical and thermal properties, thereby making flame propagation more challenging.Therefore, the comparable combustion speeds of the PUR_AD2 and PUR_AD3 samples are attributed to their similar surface area and pore volume, as previously mentioned, despite their differing morphologies.Therefore, a SiO2 content of 2 wt.% of SiO2_AD is deemed the optimal particle concentration for incorporation into the polymer structure to minimize the flammability of aerogel samples.Following common practice in the literature for this type of materials [34,35], different images were recorded with a thermal camera where the sample emits an amount of infrared radiation as a function of its temperature (Figure S4).The images reveal the notable difference in combustion when the samples contain a SiO2-based additive (2 wt.%) extracted by AD from RH, showing the char residues generated in each case.PUR undergoes thermal degradation through a wellknown mechanism [36,37] that involves its dissociation into primary amines, olefins, and carbon dioxide.This process typically occurs when PUR is exposed to elevated temperatures, leading to the breakdown of its polymeric structure (Figure S5).With the addition of a SiO2-based additive, the samples tend to form an intumescent char layer after combustion.The high quality of a SiO2 layer form would act as a good physical barrier to resist the transfer of heat and combustible gases, resulting in the enhancement of flameretardant performance.However, as other authors have observed, SiO2 excess could suppress the intumescent process of the carbonization layer, resulting in a decrease in the Figure 9 shows the calculated combustion velocity of the doped aerogels with AD and AD@QAS-PA particles.Unexpectedly, the incorporation of AD@QAS-PA particles into the polymeric matrix practically does not significantly slow down the combustion velocity of the sample, showing slightly lower values but very close to those of the undoped aerogel (PUR).This observation contradicts the expected improvement of the AD particles modification in the flame-retardant behavior according to other authors [39,40].This discrepancy likely arises from the density of the modified particles.Specifically, since the modified particles are denser than their non-modified counterparts (as indicated in Tables 1 and 3) due to the long alkyl silane chain of QAS, a smaller quantity of AD@QAS-PA particles fills the matrix when compared with the same mass of SiO 2 _AD.Consequently, this minimizes the potential flame-retardant effect.
To gain further insights into the flame-retardant behavior of doped aerogels, additional studies-particularly those considering an addition of SiO 2 dopants in a weight/volume ratio instead of a weight/weight ratio-should be conducted to explore the effective impact of using the modified biosilica in the polymeric matrix.In any case, this study suggests that enhancing the fire resistance of samples can be feasibly achieved by incorporating additives derived from natural and cost-effective sources, such as SiO 2 particles obtained from RH, as opposed to their respective counterparts.
The combustion study suggests that enhancing the fire resistance of samples can be feasibly achieved by incorporating additives derived from natural and cost-effective sources, such as SiO2 particles obtained from RH, as opposed to their respective counterparts.Figure 9 shows the calculated combustion velocity of the doped aerogels with AD and AD@QAS-PA particles.Unexpectedly, the incorporation of AD@QAS-PA particles into the polymeric matrix practically does not significantly slow down the combustion velocity of the sample, showing slightly lower values but very close to those of the undoped aerogel (PUR).This observation contradicts the expected improvement of the AD particles modification in the flame-retardant behavior according to other authors [39,40].This discrepancy likely arises from the density of the modified particles.Specifically, since the modified particles are denser than their non-modified counterparts (as indicated in Tables 1 and 3) due to the long alkyl silane chain of QAS, a smaller quantity of AD@QAS-PA particles fills the matrix when compared with the same mass of SiO2_AD.
Consequently, this minimizes the potential flame-retardant effect.
To gain further insights into the flame-retardant behavior of doped aerogels, additional studies-particularly those considering an addition of SiO2 dopants in a weight/volume ratio instead of a weight/weight ratio-should be conducted to explore the effective impact of using the modified biosilica in the polymeric matrix.In any case, this study suggests that enhancing the fire resistance of samples can be feasibly achieved by incorporating additives derived from natural and cost-effective sources, such as SiO2 particles obtained from RH, as opposed to their respective counterparts.

Conclusions
A novel method is proposed to produce polyurethane/silica (PUR/SiO2) aerogel composites with evenly dispersed SiO2 particles.This innovation utilizes SiO2 particles obtained from sustainable sources like rice husk through different procedures, introducing them as doping agents into the polymer structure of PUR aerogels.The physicochemical properties of both undoped and doped aerogels with different types of SiO2 particles (AD and SG) were analyzed, resulting in mechanically strengthened materials across the board.Additionally, the flammability of aerogels was investigated, showing a notable reduction in the burning rate, especially for aerogels doped with SiO2

Conclusions
A novel method is proposed to produce polyurethane/silica (PUR/SiO 2 ) aerogel composites with evenly dispersed SiO 2 particles.This innovation utilizes SiO 2 particles obtained from sustainable sources like rice husk through different procedures, introducing them as doping agents into the polymer structure of PUR aerogels.The physicochemical properties of both undoped and doped aerogels with different types of SiO 2 particles (AD and SG) were analyzed, resulting in mechanically strengthened materials across the board.Additionally, the flammability of aerogels was investigated, showing a notable reduction in the burning rate, especially for aerogels doped with SiO 2 particles extracted from acid digestion (AD).The influence of particle content on the combustion rate was confirmed, with samples containing over 2 wt.% displaying similar thermal conductivity, density, and porosity results without further enhancement.However, the functionalization of the AD particles with PA did not delay flame propagation.In summary, the doped aerogel samples significantly enhance the fire safety performance of the material, which is crucial in various sectors such as construction and automotive, with up to an eightfold increase in fire resistance when only doped with 2 wt.% of AD particles.Future work could focus on optimizing the doping process and exploring other sustainable sources for SiO 2 particles, as well as investigating additional functionalization techniques to further improve the flame-retardant properties and overall performance of the aerogels.

Synthesis Procedures 4.2.1. Extraction of SiO 2 Particles
The SiO 2 particles were obtained by acidic digestion of RH or by alkaline extraction of the husk ash, following procedures widely described in the literature [41].Both the acidic and alkaline processes yielded approximately 10% of SiO 2 .The extraction was performed as follows: -Acidic digestion: HNO 3 (0.5 L) was added to 0.5 kg of RH (previously milled to 0.5 mm), and the digestion occurred for 2 h at 60 • C. The digested husk was then filtered (mesh 85 µm) and washed with distilled water until pH ca.7.The neutralized husk was then dried at 110 • C for 12 h.The SiO 2 particles (SiO 2 _AD) were recovered after a calcination step (the temperature was raised to 700 • C at 4.5 • C/min and maintained at 700 • C for 3.5 h).-Alkaline extraction: RH was calcined at 700 • C (4.5 • C/min up to 700 • C and maintained at 700 • C for 3.5 h).Then, 10 g of RH ash was added to 0.2 L of NaOH aqueous solution (2.5 M) and refluxed at 90 • C for 3 h.The mixture was then filtered, and the supernatant was recovered and neutralized with HCl 10%.After 24 h, the obtained gel was centrifuged at 9000 rpm for 10 min and washed with water.The washing and centrifugation steps were repeated 3 times, and the obtained SiO 2 particles (SiO 2 _SG) were dried at 150 • C for 2 h.
The SiO 2 particles obtained by acidic extraction were modified with QAS and PA.In brief, a dispersion (1.5 wt.%) of SiO 2 _RH_AD particles in EtOH:H 2 O (1:1) was prepared.PA was added to the mixture, and after homogenization, QAS was added dropwise (molar ratio SiO 2 _RH_AD:PA:QAS was 1:5:1.3).The reaction was kept at 23 • C for 3 h, and the as-obtained solid was centrifuged (10 min at 9000 rpm) and washed 3 times with water.After this process, the agglomerates of particles were dried at 40 • C and ground to obtain the particles as a powder (SiO 2 _AD@QAS-PA).
The main features of the synthesized SiO 2 particles are summarized in Table 4. obtained solid was centrifuged (10 min at 9000 rpm) and washed 3 times with water.After this process, the agglomerates of particles were dried at 40 °C and ground to obtain the particles as a powder (SiO2_AD@QAS-PA).
The main features of the synthesized SiO2 particles are summarized in Table 4. SiO2-doped PUR aerogels were prepared at a pilot-plant scale (using a freeze-drying unit with a production of 2 m 2 of aerogel panels per batch), following our previous reported works [42,43].The main modification involved the incorporation of a doping agent (SiO2 particles).Table 5 lists the prepared aerogel samples.obtained solid was centrifuged (10 min at 9000 rpm) and washed 3 times with water.After this process, the agglomerates of particles were dried at 40 °C and ground to obtain the particles as a powder (SiO2_AD@QAS-PA).
The main features of the synthesized SiO2 particles are summarized in Table 4. SiO2-doped PUR aerogels were prepared at a pilot-plant scale (using a freeze-drying unit with a production of 2 m 2 of aerogel panels per batch), following our previous reported works [42,43].The main modification involved the incorporation of a doping agent (SiO2 particles).Table 5 lists the prepared aerogel samples.obtained solid was centrifuged (10 min at 9000 rpm) and washed 3 times with water.After this process, the agglomerates of particles were dried at 40 °C and ground to obtain the particles as a powder (SiO2_AD@QAS-PA).
The main features of the synthesized SiO2 particles are summarized in Table 4. SiO2-doped PUR aerogels were prepared at a pilot-plant scale (using a freeze-drying unit with a production of 2 m 2 of aerogel panels per batch), following our previous reported works [42,43].The main modification involved the incorporation of a doping agent (SiO2 particles).Table 5 lists the prepared aerogel samples.SiO 2 -doped PUR aerogels were prepared at a pilot-plant scale (using a freeze-drying unit with a production of 2 m 2 of aerogel panels per batch), following our previous reported works [42,43].The main modification involved the incorporation of a doping agent (SiO 2 particles).Table 5 lists the prepared aerogel samples.
PEG and DMPA were melted together with SiO 2 particles under vigorous mechanical stirring in a jacketed vessel at 80 • C. The amount of PEG was adjusted for each experiment based on the amount of doping agent added.Once melted, HMDI (isocyanate, 0.34 mol), NMP (anhydrous solvent, 0.17 mol), and DBTDL (catalyst, 8.86•10 −4 mol) were added, and the reaction proceeded for 3 h at 450 rpm, maintaining the same temperature.Subsequently, the temperature was lowered to 40 • C before adding EtOAc (organic solvent, 4.04 mol).
The mixture was stirred for an additional hour before adding TEA (neutralizer agent, 0.20 mol) to neutralize the DMPA acidic functionalities.This neutralization step took 30 min to achieve complete neutralization of carboxylic groups.All previous steps were carried out under a N 2 stream to create an inert atmosphere.In the next step, polymer chains were extended using a slight excess of the chain extender EDA (0.07 mol), adding another 30 min to the process.The EDA/HMDI ratio was 0.20.In the final stage of the synthesis method, water content was added, and the polymer dispersion was completed by mechanical stirring (900 rpm) for approximately 30 min.The added water content affected the solid content of the samples, fixed at 3.7 wt.%.Finally, the obtained wet gel was transferred into the freeze-drying trays with dimensions of 43.5 cm (long) × 34 cm (wide) × 4 cm (high) and subjected to a freeze-drying process involving freezing for 6 h at −40 • C, followed by 60 h at 25 • C and 200 µbar of primary drying and, finally, 10 h at 40 • C of secondary drying.

Characterization Techniques
Thermogravimetric analyses of SiO 2 particles and SiO 2 -doped polyurethane aerogels (TGA, system, Mettler Toledo, Madrid, Spain) were conducted to assess their thermal stability.Additionally, the glass transition temperature of PUR aerogels (T g ) was determined through differential scanning calorimetry (DSC, 2STARe system, Mettler Toledo, Madrid, Spain) at a heating rate of 20 K•min −1 , spanning from −80 to 80 • C. Data acquisition for both TGA and DSC experiments was performed using 2STARe V16.30 software.
The chemical structure of the samples was determined by assigning the functional groups based on vibrations obtained with an infrared spectrometer (Fourier transform infrared-attenuated total reflectance (FTIR-ATR), Spectrum Two, PerkinElmer, Waltham, MA, US) in the 500-4000 cm −1 range.
Density values of the synthesized aerogels were determined using a 3D scanner (REXCAN DS3 Silver, eQuality Tech Inc., Rochester Hills, MI, USA).The instrument generated a three-dimensional image of the sample with ezScan v 3.26 software, and the volume of the aerogel was subsequently calculated using Geometric Wrap v2021.2.2 software.Specifically, three square-shaped samples were extracted from different regions of each synthesized aerogel.The samples were previously weighted to calculate their density (ρ, g•cm −3 ) by dividing their mass by the volume obtained from the scanner.
Moreover, the thermal conductivity of the synthesized aerogels was assessed through heat transfer measurements between two parallel plates (HFM 300, Linseis, Selb, Germany).To conduct these assessments, samples with dimensions of 30 × 30 cm were used.These analyses were carried out at five intermediate temperatures, between 0 and 40 • C.
The morphological structure of the samples was examined through high-resolution scanning electron microscopy (HRSEM, GeminiSEM 500, ZEISS, Jena, Germany) using an 80 mm 2 energy dispersive spectroscopy (EDS) sensor and another electron backscatter diffraction (EBSD) sensor.Thus, SEM images, elemental mapping, and EDS spectrum were obtained.
To analyze the pore size distribution (PSD) of the aerogels, a PoreMaster mercuryintrusion apparatus (Quantachrome Instruments, Boynton Beach, FL, USA) was employed.The method relies on the infiltration of mercury into the pores of the samples under controlled pressure, enabling penetration.Subsequently, the registration of mercury volume, correlated with increasing pressure, allows the determination of the pore size distribution [44].
Finally, the flame-retardancy behavior of the synthesized aerogels was directly assessed in the absence and presence of the different SiO 2 particles (AD, SG, and AD@QAS-PA).The flame spread across vertically oriented aerogels (ca. 10 × 3 cm) was evaluated according to [45].The ignition source was applied to the aerogel surface for 3 s, followed by a waiting period of 10 s to measure the subsequent flame spread.Each analysis was carried out with triplicate samples (n = 3).

Figure 4 .
Figure 4. Combustion velocity of the synthesized SiO2-doped PUR aerogels (PUR_AD2 and PUR_SG2) and the corresponding pictures of PUR aerogels before and after the burning process.

Figure 4 .
Figure 4. Combustion velocity of the synthesized SiO 2 -doped PUR aerogels (PUR_AD 2 and PUR_SG 2 )and the corresponding pictures of PUR aerogels before and after the burning process.

Figure 5 .
Figure 5. SEM micrographs corresponding to the different PUR_AD aerogels.Figure 5. SEM micrographs corresponding to the different PUR_AD aerogels.

Figure 5 .
Figure 5. SEM micrographs corresponding to the different PUR_AD aerogels.Figure 5. SEM micrographs corresponding to the different PUR_AD aerogels.

Figure 6 .Table 3 . 5 *
Figure 6.Pore size distribution corresponding to the different PUR_AD aerogels.Table 3. Characterization results of the SiO 2 -doped PUR aerogels with particles derived from acidic extraction.

Figure 7 .
Figure 7. (A) Combustion velocity corresponding to the aerogel synthesized using different wt.% of AD_SiO 2 particles; (B) pictures of polyurethane aerogels before (left) and after (right) the burning process.

Figure 8 .
Figure 8. SEM micrographs corresponding to PUR aerogels doped with SiO2 particles derived from acidic extraction.

Figure 8 .
Figure 8. SEM micrographs corresponding to PUR aerogels doped with SiO 2 particles derived from acidic extraction.

Figure 9 .
Figure 9. Combustion velocity of the synthesized SiO2-doped PUR aerogels (PUR_AD2 and PUR_AD@QAS-PA2) and the corresponding pictures of PUR aerogels before and after the burning process.

Figure 9 .
Figure 9. Combustion velocity of the synthesized SiO 2 -doped PUR aerogels (PUR_AD 2 and PUR_AD@QAS-PA 2 ) and the corresponding pictures of PUR aerogels before and after the burning process.
Experiments labeled 1 and 6 are identical. (** ) SiO 2 _AD x : silica obtained by acidic-extraction-based PUR aerogel containing x wt.% of silica related to the total amount of PUR.SiO 2 _AD@QAS-PA x : silica obtained by acidic extraction and functionalized with dimethyloctadecyl(3-(trimethylsilyl)propyl ammonium chloride and phytic-acid-based PUR aerogel containing x wt.% of silica related to the total amount of PUR.SiO 2 _SG x : silica obtained by alkaline extraction, followed by sol-gel-precipitation-based PUR aerogel containing x wt.% of silica related to the total amount of PUR.
* Obtained by a Hg porosimeter.

Table 2 .
Characterization results of the SiO 2 -doped PUR aerogels with different particle contents.
* Obtained by a Hg porosimeter.Gels 2024, 10, 465 6 of 16 Gels 2024, 10, x FOR PEER REVIEW 6 of 18 polymer materials by suppressing the intense bubbling phenomenon that occurs during the degradation process in combustion [30,31].

Table 2 .
Characterization results of the SiO2-doped PUR aerogels with different particle contents.

Table 4 .
Main characteristics of the extracted SiO 2 particles from rice husk.

Table 4 .
Main characteristics of the extracted SiO2 particles from rice husk.

Table 5 .
SiO2 nature and content of the final doped PUR aerogels.

Table 4 .
Main characteristics of the extracted SiO2 particles from rice husk.

Table 5 .
SiO2 nature and content of the final doped PUR aerogels.

Table 4 .
Main characteristics of the extracted SiO2 particles from rice husk.

Table 5 .
SiO2 nature and content of the final doped PUR aerogels.

Table 5 .
SiO 2 nature and content of the final doped PUR aerogels.